immutable promise broken in unions?

[Shriramana Sharma via Digitalmars-d-learn]

John Colvin wrote:

> Casting away immutable can sometimes be necessary (e.g. when
> talking to other languages), so I'm not sure it should be
> disallowed, but it'd be great if it was somehow easier to catch
> these bugs.

Yes it was in the context of talking to C that I needed to make such a 
union. But already making a union is breaking the type system in a way, no?

-- 
Shriramana Sharma, Penguin #395953